Recently, various types of materials have been studied as electrode material for energy storage devices. Among the materials, lithium titanate is attracting attention for its superior input-output performance when used as an active material. Further, when using as an energy storage device to be mounted on a vehicle, since it will be used in extremely cold regions, development of an active material with excellent input performance at very low temperature like −30° C. is desired.
Patent Document 1 discloses lithium titanate containing Li4/3Ti5/3O4 as a main component, having a main peak intensity of 5 or less in each of anatase-type titanium dioxide, rutile-type titanium dioxide, and Li2TiO3 when the intensity of the main peak for Li4/3Ti5/3O4 determined by X-ray diffraction analysis using Cu target is defined to be 100, and with high crystallinity having a crystallite diameter of 700 Å to 800 Å (70 nm to 80 nm), which is determined from a half-peak width of the peak in 4.83 Å with the Scherrer equation. According to Patent Document 1, high initial charge-discharge capacity is achieved by the peak intensity being as small as 5 or less in lithium titanate components other than Li4Ti5O12 and very high crystallinity having a crystallite diameter of within the range of 70 nm to 80 nm.
Patent Document 2 discloses a lithium-titanium complex oxide containing Li4Ti5O12 as a main component, in which when the main peak intensity detected for each phase of Li4Ti5O12, Li2TiO3, and TiO2 in an X-ray diffraction pattern is I1, I2, and I3 respectively, achieving I1/(I1+I2+I3) of 96% or more, having a crystallite diameter, which is determined from the half-peak width of the peak in the X-ray diffraction pattern for the (111) plane of Li4Ti5O12 with the Scherrer equation, of 520 Å to 590 Å (52 nm to 59 nm), preferably having a ratio of volume surface diameter determined by the BET method to the crystallite diameter, that is volume surface diameter/crystallite diameter, of 4 or less, and further preferably, having specific surface area determined by the BET method of 8 to 12 m2/g and maximum primary particle diameter of 1.5 μm or less. According to Patent Document 2, lithium-titanium complex oxide having a crystallite diameter within the above range can form fine particles while having high crystallinity, and is considered to exhibit high-rate performance while having a high initial capacity of 160 mAh/g for example.
Patent Document 3 discloses a lithium-titanium complex oxide containing Li4Ti5O12 as a main component, having a crystallite diameter of 690 Å or less, and wherein, when the main peak intensity of spinel type lithium titanate by the X-ray diffraction analysis is 100, the main peak intensities of rutile-type TiO2, anatase-type TiO2, and Li2TiO3 are all 7 or less. According to Patent Document 3, as the crystallite diameter is made smaller and impurity phases are made fewer, diffusion speed of lithium ion becomes faster and ionic conductivity of lithium improves, thus it is said that high input performance at room temperature is realized.
Patent Document 4 discloses a lithium-titanium complex oxide that has a spinel-type structure and that is represented with a compositional formula Li[Li(1-2x)/3MgxTi(5-x)/3]O4(0.025≦x≦0.063). According to Patent Document 4, by replacing a part of titanate site of lithium titanate by magnesium, electron conductivity of active material improves and high output performance is achieved at −30° C.